posted
On track where concrete ties are used, why are wood ties are still used for switches?
I thought it was because the old wood ties were replaced with concrete ties, but their machines couldn't get under the switches-so they left the wood ties in place under the switch.
However, BNSF recently finished a 3rd mainline on the San Bernardino Sub here in Southern California, and while concrete ties were used, brand-new wood ties were placed underneath the crossovers.
???
George Harris Member # 2077
posted
Anyone laying new concrete ties and leaving the OLD switchties in place will quickly regret it.
Why use wood switchties in a line when you install concrete ties? Primarily cost.
Currently a concrete tie with springclips costs more, but not a lot more than a good wood tie with tieplates, spikes, and anchors.
However, when we get to switchties, the cost difference is huge. As a consequence, for a lot of installations the estimated savings in maintenance cost of a concrete tie turnout versus a turnout on wood switch ties is not worth it.
The major reason for the huge cost difference is that on normal concrete ties, the rail position is identical on every one, and they can be churned out by the thousands, all alike.
For switchties, each one has a different rail mounting position, and there are also special mountings required for switch slide plates, stock rail braces, frog mounting, and special fastenings where the rails are very close together. The longest switchtie is around 16 feet long, and is approximately twice the weight of a standard concrete tie. Installation cost is also quite a bit higher than for a turnout on wood ties. Each tie must be very precisely located. When you are dealing with wood ties, for say a 100 foot long turnout with average 20 inch tie spacing, you would have 60 wood ties, so many a 9 feet, so many at 10 feet, so many ay 11 feet, etc, up to a few 16 footers. Some railroads may do their lengths in six inch increments, but still you have either 8 different length or no more than 16 different lengths. With a set of concrete switchties, you have 60 unique pieces of concrete that must be placed in the right order and at precise spacing.
In high speed lines where small differences in track spring constant can have significant impacts on ride quality, the ride quality issue usually trumps the cost issue. In very high volume tracks, since maintenance is usually directly related to passing tonnage, the payback on the maintenance savings may be fast enough and large enough to say go concrete, but at average speeds and average traffic volumes, the answer is usually no. Also, in transit systems, with their very light axle loads, the economic answer is also usually no. Sometimes placement of the turnout in a difficult to maintain location will also affect the conclusion.
My bill for professional services is in the mail.
George
rY. Member # 3528
posted
Typos or no, I take issue with any comparison between George's thorough replies and "rotted wooden ties", sheesh!
Kiernan Member # 3828
posted
If you're placing concrete ties along a track, cost of ties is probably not an important consideration for switches. For all the reasons that George stated above, switches tend to be pre-manufactuered, you get much better quality control. A precast concrete firm could easily make the jig necessary for switch manufacture, after all, they'd only have to make it once.
It would be extremely difficult to design a concrete tie that would take the lifting and placing loads without cracking. The load on a tie is approximately equivalent to an upside-down, uniformly loaded, simply supported beam. The tie has two point loads where the rails attach and a uniformly distributed reaction where it meets the ballast. A concrete tie is designed for that load, whether it's reinforced concrete or prestressed concrete. Concrete is a fine material, and correctly made concrete ties could last almost forever. But with pre-manufacture, you must consider how the completed piece is going to get to the jobsite. In this case, timber is a much better material for pre-manufacture. It can withstand different loading conditions that it would receive during shipping and placing.
George Harris Member # 2077
posted
The basic specification for Concrete Ties in the American Railway Engineering and Maintenance of Way Manual (2005 update, I have not yet gotten my 2006) runs to 57 pages. (Chapter 30, Part 4) This is actually a general specification, covering quite a few possibilities, so if you want to buy some, you usually end up writing a few more pages of specification to claify what exact properties and other things you do want and don't want. It does cover concrete switchties, as well.
Concrete ties are usually prestressed, and cast in long lines with the prestressing continuous over quite a few ties. There are end headers set in the forms between ties, so that when you demold them all you have to cut is the prestressing tendonds, not the concrete. The usual concrete strength is 7000 psi or higher. The design rules are somewhat different than those for concrete beams. there are some that are post tensioned, but I don't think anybody in the US uses them.
There are also two block ties with a steel bar between two short concrete blocks, one under each rail seat. These are reinforced blocks. They are or were the standard in France for many years, and were also a standard in teh past in Mexico, but I have heard that the Mexicans did not have a consistent good experience with them.
posted